Aqueous hydrogen peroxide solutions and method of making same

ABSTRACT

An aqueous hydrogen peroxide solution containing i) less than 50 wppm alkali metals, alkaline earth metals or combinations thereof in total, irrespective whether the alkali or alkaline earth metals are present in cationic or complex form; ii) less than 50 wppm of amines having a pk B  of less than 4.5 or the corresponding protonated compounds in total; and iii) at least 100 wppm anions or compounds that can dissociate to form anions in total, where the wppm are based on the weight of hydrogen peroxide and the concentration of hydrogen peroxide is more than 50% by weight based on the total weight of the hydrogen peroxide solution. A process for preparation of said hydrogen peroxide solution and the use of said solution in a process for epoxidation of olefins is also disclosed.

This application claims the benefit of our provisional application No.60/414,327 filed Sep. 30, 2002 which is relied and incorporated hereinby reference.

INTRODUCTION

The present invention relates to specific aqueous hydrogen peroxidesolutions that are characterized by a maximum amount of alkali metals,alkaline earth metals, and amines having a pk_(B) of less than 4.5, andthat are particularly suitable for use in processes for the epoxidationof olefins. In another aspect, the present invention relates to aprocess for the preparation of such an aqueous hydrogen peroxidesolution.

BACKGROUND OF THE INVENTION

Today, the vast majority of hydrogen peroxide is produced by thewell-known anthraquinone process. A survey of the anthraquinone processand its numerous modifications is given in G. Goor, J. Glenneberg, S.Jacobi: “Hydrogen Peroxide” Ullmann's Encyclopedia of IndustrialChemistry, Electronic Release, 6^(th)ed. Wiley-VCH, Weinheim June 2000,page 14. Generally, the anthraquinone loop process comprises thefollowing steps:

(a) Hydrogenation of a working solution comprising an organic solvent ormixture of organic solvents, and one or more active anthraquinonecompounds;

(b) oxidation of the resulting hydrogenated working solution to formhydrogen peroxide;

(c) extraction of hydrogen peroxide with water;

(d) stabilizing of the extracted aqueous hydrogen peroxide solution;

(e) drying of the working solution after extraction; and

(f) regeneration and purification of the working solution.

For each of the above distinct process steps, the Ullmann referencediscloses numerous different possibilities.

Crude hydrogen peroxide solutions or concentrated hydrogen peroxidesolutions obtained from the anthraquinone process contain a plurality ofcompounds in addition to hydrogen peroxide in low concentrations. Thesecompounds are either impurities or additives like stabilizers. Theimpurities are compounds that are extracted from the working solutioninto the aqueous phase. They are mainly ionic or polar species likecarboxylic acids, alcohols, carbonyl compounds and amines. Theseimpurities are therefore also found in commercial hydrogen peroxidesolutions.

For example, hydroquinone solvents that are commonly used in the abovedescribed process are nitrogen containing compounds like amides andureas (see Ullmann supra page 6). Particularly preferred are tetraalkylureas like tetrabutyl urea. The use of these solvents result in amineimpurities like monoalkyl or dialkyl, especially monobutyl and dibutyl,amines in the final hydrogen peroxide solutions. For example, thecommercial hydrogen peroxide solution HYPROX® available from Degussa AGcontains up to 200 wppm mono- and dibutyl amine based on the weight ofhydrogen peroxide.

Depending on the final use of the hydrogen peroxide solutions, it isalso known to conduct additional purification steps in order to obtainthe required specification for the respective use of the hydrogenperoxide solution.

For example, DE-A 100 26 363 discloses a purification process foraqueous hydrogen peroxide solutions, whereby the solutions are treatedwith an anion exchange resin, a nonionic absorbing resin having aspecific structure, and a neutral absorbing resin also having a specificmacroporous structure. The hydrogen peroxide solutions obtained in thisway are substantially free of cationic, anionic and organic impurities.Therefore, the solutions are particularly useful in microelectronicsapplications.

Similarly U.S. Pat. No. 4,999,179 discloses a process for purificationof hydrogen peroxide solutions that contain, after purification, eachmetal cation in an amount of less than 5 ppb, each anion in an amount ofless than 10 ppb and organic impurities in an amount of not more than 5ppm in terms of total organic carbon content.

The drawback of such methods is that the purification is extremelyexpensive and can therefore, for economic reasons, not be used for thepreparation of chemical mass products like propylene oxide. Furthermore,such highly purified hydrogen peroxide solutions are substantially freeof anionic components like phosphates and nitrates that are necessaryfor the stabilization of aqueous—especially highly concentrated—hydrogenperoxide solutions for safety reasons.

From EP-A 100 119, it is known that propene can be converted by hydrogenperoxide into propene oxide if a titanium-containing zeolite is used ascatalyst.

Since then, many investigations with respect to the effect of theaddition of basic, acidic and ionic compounds either during preparationof the titanium silicalite catalyst or their presence in the reactionmixture on the activity and selectivity of the catalysts have beenpublished.

From EP-A 230 949, it is known to neutralize the titanium silicalitecatalyst either prior to its use in an epoxidation reaction or in situwith strong bases thereby introducing large amounts of alkali metal oralkaline earth metal ions into the reaction mixture. Said neutralizationresulted in an increase in activity and selectivity to the desiredolefin oxide in a batch process.

The experiments in EP-A 757 043, however, show that in a continuousprocess the activity is considerably reduced if the catalyst isneutralized prior to or during the reaction. Therefore, it is suggestedto treat the catalyst prior to or during the epoxidation reaction with aneutral or acidic salt. The experimental data in EP-A 757 043 confirmthat by addition of neutral or acidic salts the selectivity is increasedbut the activity is less reduced compared to the addition of a base. ButEP-A 757 043 only shows examples wherein the catalyst is treated withthe salt prior to the reaction and the catalyst is used in slurry form.Additionally, the experiments were only run for 8 hours but neverthelessshow a dramatic drop in catalyst activity only after 4 hours, which isby no means acceptable for an industrial process.

Similarly, EP-A 712 852 teaches that by performing an epoxidationprocess catalyzed by titanium silicalite in the presence of a non-basicsalt the selectivity is increased. All the examples are run in batchoperation mode with a stirred catalyst slurry for one hour. Although itcan be confirmed that the presence of non-basic salts may have apositive influence on catalyst selectivity in a short term experiment,it was discovered that even if non-basic salts are present in a reactionmixture for a continuous epoxidation reaction the activity andselectivity drops dramatically over time. Thus, the teaching of EP-A 712852 does not lead to a reaction system that can be economically employedin a continuous epoxidation process using hydrogen peroxide in thepresence of a heterogeneous catalyst.

In WO 00/76989, the influence of ionic components in commerciallyavailable aqueous hydrogen peroxide solutions that are used inepoxidation reactions as described in the above prior art documents isdiscussed. Ionic components, especially phosphates and nitrates, areadded to commercially available aqueous hydrogen peroxide solutions asstabilizers to reduce hazardous decomposition of hydrogen peroxide.Contrary to the disclosure in the above prior art documents, WO 00/76989teaches that the presence of ionic components in the reactionmixture—even those that have been added as stabilizers to commercialhydrogen peroxide—is detrimental to the long term selectivity in acontinuous titanium silicalite catalyzed epoxidation reaction and shouldtherefore be reduced to a minimum.

Contrary to the above prior art documents, continuous reactions runningup to 300 hours were conducted showing that if ionic components arepresent in an amount of more than 100 ppm the long term selectivity isreduced. To solve this problem, it is suggested to remove ioniccomponenets from hydrogen peroxide solutions prior to use in epoxidationreactions by means of ion exchangers. Moreover, WO 00/76989 teaches thatammonium compounds and ammonia should be avoided under any circumstancessince these compounds may lead to undesired side products by oxiranering opening reactions with the formed olefin oxide. Although theteaching in WO 00/76989 leads to some improvement in long termselectivity compared to the above art, this improvement is stillinsufficient for an industrial scale process. Furthermore, thisimprovement can only be achieved with the complicated and, both in termsof investment and process costs, economically undesirable additionalprocess step of ion exchange. Last but not least, removal of stabilizingions like phosphate and nitrate from the hydrogen peroxide solutionmakes the process more hazardous and additional measures have to betaken to ensure safety during the entire process.

Contradicting the teaching of WO 00/76989, WO 01/57012 discloses thatthe use of crude hydrogen peroxide solutions directly obtained from theanthraquinone process having large amounts of, for example, sodium,nitrate, phosphate, and organic impurities, is superior with respect toproduct selectivity compared to highly purified hydrogen peroxidesolutions containing very low amounts of sodium, nitrate, and phosphate.The experiments, however, were only conducted for a few hours so thatthe long term activity and selectivity of the catalyst cannot bedetermined from that reference.

Again another approach is disclosed in WO 01/92242, wherein a titaniumsilicalite catalyzed process for epoxidation of olefins using crudehydrogen peroxide solutions in the presence of a compound havingaminocarbonyl functionality in which the nitrogen bears at least onehydrogen atom is disclosed. The examples show a batch type process thatis conducted up to a conversion of hydrogen peroxide of 85%. After twohours the reaction is terminated even if the conversion of 85% has notbeen reached. Although the experimental data show an improvement withrespect to the reaction rate compared to compounds with aminocarbonylfunctionality having no hydrogen atom bonded to the nitrogen atom, longterm activity and selectivity of the catalyst in a continuous process isnot determinable from the information in WO 01/92242.

DE-A 199 36 547 discloses a continuous titanium silicalite catalyzedprocess for epoxidation of olefins with hydrogen peroxide whereby theconversion is kept constant by increase of reaction temperature andadjusting the pH of the reaction mixture. In a long term experiment(1000 hours), it could be verified that by adjusting the pH the increasein temperature and the rate of increase could be reduced compared to anexperiment without pH adjustment. But conversion and selectivity werethe same, irrespective of whether the pH was adjusted or not.

Thus, the object of the present invention is to provide an aqueoushydrogen peroxide solution that can be economically produced, that canbe safely handled, stored, and shipped, and that is suitable for theepoxidation of olefin in the presence of a heterogeneous catalyst andleads to improved long term activity and selectivity of the catalyst.

SUMMARY OF THE INVENTION

In carrying out the present invention there is prepared an aqueoushydrogen peroxide solution comprising:

i) less than 50 wppm of a member selected from the group consisting ofan alkali metal, an alkaline earth metal or combinations thereof intotal, irrespective whether the alkali or alkaline earth metals arepresent in cationic or complex form;

ii) less than 50 wppm of amines having a pk_(B) of less than 4.5 or thecorresponding protonated compounds in total; and

iii) at least 100 wppm anions or compounds that can dissociate to formanions in total,

whereby the wppm are based on the weight of hydrogen peroxide.

This inventive aqueous hydrogen peroxide solution can be obtained by aprocess for the preparation of the hydrogen peroxide solution accordingto the anthraquinone loop process comprising:

(a) hydrogenation of a working solution comprising an organic solvent ormixture of organic solvents and one or more active anthraquinonecompounds,

(b) oxidation of the resulting hydrogenated working solution to formhydrogen peroxide,

(c) extraction of hydrogen peroxide with water,

(d) stabilizing of the resulting extracted aqueous hydrogen peroxidesolution,

(e) concentrating the aqueous hydrogen peroxide solution to aconcentration of hydrogen peroxide of at least 50% by weight based onthe weight of the hydrogen peroxide solution,

(f) drying of the working solution after extraction, and

(g) regeneration and purification of the working solution, and duringthe entire process neither alkali or alkaline earth metals nor amineshaving a pk_(B) of less than 4.5 or compounds forming such amines duringthe process are introduced in amounts that result in amounts of

-   -   i) 50 wppm or more of alkali metals, alkaline earth metals or        combinations thereof in total, irrespective whether the alkali        or alkaline earth metals are present in cationic or complex        form; or    -   ii) 50 wppm or more of amines having a pk_(B) of less than 4.5        or the corresponding protonated compounds in total;    -   in the resulting aqueous hydrogen peroxide solution, where the        wppm are based on the weight of hydrogen peroxide.

The hydrogen peroxide solution of the present invention is particularlysuitable for use in a process for the epoxidation of olefins in thepresence of a heterogeneous catalyst. It is a surprising result of thepresent invention that a hydrogen peroxide solution fulfilling theabove-specified requirements and that can be safely handled, stored, andshipped, can easily be prepared in an economical process. Furthermore,surprisingly, this aqueous hydrogen peroxide solution leads to animproved long term activity and selectivity of the heterogeneouscatalyst in an epoxidation process. Consequently, the overall economicsof an epoxidation process can be considerably improved using theinventive aqueous hydrogen peroxide solution, since the solution itselfcan be economically produced and leads to reduced deactivation of thecatalyst so that the operation time between regeneration cycles in theepoxidation process can be increased.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have thus discovered, contrary to the teaching ofthe prior art, that the presence of alkali metals and alkaline earthmetals above a certain limit are detrimental to the activity andselectivity of the catalyst employed in epoxidation reactions ofolefins. Moreover, the inventors have recognized that—in addition toalkali metals and alkaline earth metals—amines having a pk_(B) of lessthan 4.5 are even more detrimental to the activity and selectivity ofthe catalyst, and therefore their content in hydrogen peroxide solutionsthat are used in epoxidation reactions of olefins has to be carefullycontrolled to be below the specified limits. On the other hand, anionslike phosphate or nitrates, that are frequently used to stabilizeaqueous hydrogen peroxide solutions, have no or only very little effecton the activity and selectivity of the epoxidation catalyst. Since theseanions are necessary for the stabilization in order to ensure safety ofhandling, storing, and shipping of the aqueous hydrogen peroxidesolution, they should be present in stabilizing amounts of at least 100wppm based on the weight of the hydrogen peroxide in the solution.

Contrary to the teaching of the prior art, neither the use of crudehydrogen peroxide solutions obtained from the anthraquinone processwithout carefully controlling the amount of alkali metals and amineshaving a pk_(B) below 4.5, nor the use of purified hydrogen peroxidesolutions, where in addition to the metal cations also the stabilizinganions have been removed, are suitable for an economical process forepoxidation of olefins.

Although an amount of alkali metals or alkaline earth metals of lessthan 50 wppm based on the weight of hydrogen peroxide in the solution isacceptable, it is preferred to reduce the amount of these components tobe less than 40 wppm, more preferred less than 35 wppm, in order tofurther improve the long term activity and selectivity of the catalyst.

So far, in the literature, the detrimental effect on amines having apk_(B) of less than 4.5 on the long term selectivity and activity of anepoxidation catalyst has not been recognized.

The effect of the presence of such amines is even more pronounced thanthe effect of the alkali metals or alkaline earth metals. Therefore, itis particularly preferred to reduce the amount of amines having a pk_(B)of less than 4.5 in the aqueous hydrogen peroxide solution in total toless than 40 wppm, preferably less than 30 wppm, more preferably lessthan 20 wppm, and most preferably less than 10 wppm, based on the weightof hydrogen peroxide in the solution.

Especially detrimental to the activity and selectivity of theepoxidation catalyst is the presence of alkyl amines, especiallysecondary and tertiary alkyl amines.

Another surprising result of the inventors' investigations is thatalthough amines having a pk_(B) below 4.5 above certain amountsdramatically reduce the long term activity and selectivity of theepoxidation catalyst, the addition of at least 100 wppm of bases havinga pk_(B) of at least 4.5 even improve the long term activity andselectivity of the epoxidation catalyst. Thus, according to a preferredembodiment of the present invention, the aqueous hydrogen peroxidesolution contains in addition at least 100 wppm of bases having a pk_(B)of at least 4.5, or the corresponding protonated compounds in totalbased on the weight of hydrogen peroxide.

These bases may be either introduced during the process for preparationof the hydrogen peroxide or may be added to the hydrogen peroxidesolution at any stage between production of the solution and final usein the epoxidation reaction.

Such bases are preferably present in the hydrogen peroxide solution inan amount of at most 3000 wppm in total, more preferred from 150 to 2000wppm, particularly preferred from 200 to 1500 wppm, and most preferredfrom 300 to 1200 wppm, based on the total weight of hydrogen peroxide.

Such bases are preferably selected from organic amines and amides havinga pk_(B) of at least 4.5, organic hydroxylamines having a pk_(B) of atleast 4.5, ammonia and hydroxylamine. Ammonia is particularly preferred.

It is a particular advantage of the hydrogen peroxide solutions of thepresent invention that anions can be present in the usual stabilizingamounts. These stabilizing anions are preferably any kind ofoxophosphorous anions like orthophosphate, hydrogen phosphate,dihydrogen phosphate, pyrophosphate, nitrate.

These stabilizing anions, or compounds that can dissociate in thehydrogen peroxide solution to produce these stabilizing anions, arepreferably present in an amount of at most 1000 wppm, preferably100-1000 wppm, more preferred 200-800 wppm, most preferred 200-600 wppm,based on the weight of hydrogen peroxide.

Thus, the hydrogen peroxide solution of the present invention ensureshigh selectivity and activity of a catalyst in the epoxidation reactionwithout compromising safety when handling, storing, and shipping thehydrogen peroxide solution.

Another advantage of the hydrogen peroxide solution of the presentinvention is that it can be easily produced in an economical wayemploying the well-known anthraquinone process, whereby additionalpurification steps are not necessary and are preferably not carried outwhen conducting the process of the present invention. The onlyrequirement for the process of the present invention compared to theknown modifications of the anthraquinone process is that the process hasto be carefully controlled to avoid introduction of alkali metals,alkaline earth metals, amines having a pk_(B) of less than 4.5, orcompounds that may form, during the anthraquinone process, such aminesduring the preparation of the hydrogen peroxide solution in amounts thatwould result in concentrations above the limits specified according tothe present invention.

Although many variations of the anthraquinone process to achieve thisrequirement are conceivable, it is particularly preferred to use aworking solution that is essentially free of organic nitrogen compounds,to dry the working solution in above step (f) without using alkalimetals or alkaline earth metal compounds that are in the anthraquinoneprocess of the prior art commonly employed for drying, and to regeneratethe working solution in step (g) by treating with active aluminum oxide.Preferably, drying is conducted by water evaporation in vacuum.

Thus, the process of the present invention provides the inventivehydrogen peroxide solution that is particularly useful in epoxidationreactions without employing cost- and labor-intensive purificationsteps. It follows that a crude hydrogen peroxide solution obtained fromthe process of the present invention can be used directly without anyfurther purification steps.

It is preferred to concentrate the hydrogen peroxide solution to ahydrogen peroxide concentration of more than 50% by weight, preferablymore than 60% by weight, most preferred from 60 to 70% by weight, basedon the total weight of the hydrogen peroxide solution. The inventorshave recognized that such concentrated hydrogen peroxide solutions areparticularly useful in the epoxidation reaction since they furtherimprove the long term activity and selectivity of the catalyst.

The hydrogen peroxide solution of the present invention can be employedin any epoxidation reaction using hydrogen peroxide known in the art. Itis particularly preferred to use the present hydrogen peroxide solutionin a continuous epoxidation process conducted in the presence of awater-miscible solvent and a heterogeneous catalyst. Preferably, thesolvent is methanol, the olefin is propene, and the heterogeneouscatalyst is a titanium silicalite catalyst.

The invention will now be explained in more detail with reference to thefollowing examples.

EXAMPLES Example 1

Preparation of an aqueous hydrogen peroxide solution according to thepresent invention.

In a trial plant for the loop process according to the anthraquinoneprocess for the preparation of hydrogen peroxide comprising the steps ofhydrogenation, oxidation, extraction, drying, and regeneration, aworking solution is used comprised of 0.11 mol/l 2-ethyl anthraquinone,0.29 mol/l 2-ethyl tetra-hydroanthraquinone, 0.13 mol/l 2-isohexylanthraquinone, and 0.12 mol/l 2-isohexyl tetra-hydroanthraquinone in asolvent mixture comprising 75 vol % of C₉/C₁₀ alkyl substituted arylcompounds, and 25 vol % of tris(2-ethyl hexyl) phosphate. In thehydrogenation step, a loop reactor was run at a hydrogen pressure of0.35 and a temperature of 58° C. Palladium black (0.5:1 g/l) was used ashydrogenation catalyst. The hydrogen peroxide equivalent in thehydrogenation was 13.0 g/l.

After the hydrogenation, a part of the hydrogenated working solution isregenerated using active aluminum oxide. Thereafter, the combinedworking solution is oxidized using the Laporte oxidation as described inUllmann, supra, page 14. Thereafter, the hydrogen peroxide is extractedusing deionized water. To the extraction water, 50 ppm H₃PO₄ and 20 ppmHNO₃, both based on the weight of the hydrogen peroxide were added. Theconcentration of the extracted aqueous hydrogen peroxide solution was41%. The working solution was dried by water evaporation in vacuum, andthereafter recycled to the hydrogenation step. The crude hydrogenperoxide solution was stabilized using 200 ppm sodium pyrophosphatebased on the weight of hydrogen peroxide and concentrated in vacuum bywater evaporation.

The hydrogen peroxide concentration of the solution obtained in this waywas 43 wt-%, based on the total weight of the solution, and contained250 mg/kg H₂O₂ phosphates, 20 mg/kg H₂O₂ nitrate, and 30 mg/kg H₂O₂ ofsodium.

Examples 2 to 5 and Comparative Examples 1 to 3

The hydrogen peroxide solution obtained from Example 1 is concentratedto a hydrogen peroxide concentration as indicated in Table 1.

Additionally, alkali metal ions and/or amines having a pk_(B) of lessthan 4.5 are added as indicated in Table 1. Furthermore, ammonia isadded in an amount of 500 wppm (1000 wppm ammonia in example 5), basedon the weight of hydrogen peroxide.

A titanium silicalite catalyst was employed in all examples. Thetitanium silicalite powder was shaped into 2 mm-extrudates using asilica sol as binder in accordance with Example 5 in EP-A 1 138 387.

Epoxidation is carried out continuously in a reaction tube of 300 mmvolume, a diameter of 10 mm, and a length of 4 m. The equipment furthercomprises three containers of liquids and relevant pumps and a liquidseparation vessel. The three containers for liquids contained methanol,the hydrogen peroxide solution, and propene. The reaction temperature iscontrolled via an aqueous cooling liquid circulating in a coolingjacket, whereby the cooling liquid temperature is controlled by athermostat. The reaction pressure was 27 bar absolute. Mass flow of thefeeding pumps were adjusted to result in a propene concentration of 38wt-%, a methanol feed concentration of 48.7 wt-%, and a hydrogenperoxide feed concentration of 8 wt-%. The reactor was operated indown-flow operation mode. The cooling jacket temperature was adjusted to35° C. and total mass flow was 0.35 kg/h. Product output and propeneoxide concentration were determined by gas chromatography, and thehydrogen peroxide conversion by titration. The selectivity of hydrogenperoxide with respect to propene oxide was calculated.

The results are given in Table 1.

TABLE 1 Addition H₂O₂ Running SH₂O₂ Exam- [mg/kg concentration timeCH₂O₂ to PO ple H₂O₂] [wt-%] [h] [%] [%] 2 — 60  649 95 91 3 Na 25 70 754 95 90 4 Li 25 60  988 94 89 5 — 60 2356 94 90 C1 Na 20; dibutyl 432083 26 72 amine 135 C2 methyl amine 60 1193 22 81 100 C3 170 60 1007 8979

In Table 2, the pk_(B) values for nitrogen-containing bases are given.

TABLE 2 Bases pk_(B) Ammonia 4.76 Methyl amine 3.36 Dibutyl amine 2.75

As is evident from the experimental results summarized in Table 1, highhydrogen conversions and selectivities can be maintained for a longrunning time of the experiment if the alkali metal concentration isbelow 50 wppm, based on the weight of hydrogen peroxide. When looking tothe comparative examples, it becomes evident that if the claimed limitsfor alkali metal ions and amines having a pk_(B) below 4.5 are exceeded,the conversion as well as the selectivity of the catalyst dramaticallydrops over time.

Further variations and modifications of the foregoing will be apparentto those skilled in the art and are intended to be encompassed by theclaims appended hereto.

1. An aqueous hydrogen peroxide solution comprising: less than 50 wppmalkali metals, alkaline earth metals or combinations thereof in total,irrespective whether the alkali metals or alkaline earth metals arepresent in cationic or complex form; less than 50 wppm of amines havinga pk_(B) of less than 4.5 or the corresponding protonated compounds intotal; and at least 100 wppm anions or compounds that can dissociate toform anions in total, the wppm being based on the weight of hydrogenperoxide, wherein the concentration of hydrogen peroxide is more than50% by weight based on the total weight of the hydrogen peroxidesolution.
 2. The aqueous hydrogen peroxide solution of claim 1, whereinthe concentration of hydrogen peroxide is more than 60% by weight basedon the total weight of the hydrogen peroxide solution.
 3. The aqueoushydrogen peroxide solution of claim 1, wherein the concentration ofhydrogen peroxide is from 60 to 70% by weight by weight based on thetotal weight of the hydrogen peroxide solution.
 4. A process for thepreparation of a hydrogen peroxide solution according to theanthraquinone loop process, said process comprising: a) hydrogenating aworking solution comprising an organic solvent or mixture of organicsolvents and one or more active anthraquinone compounds to obtain ahydrogenated working solution, b) oxidizing the hydrogenated workingsolution to form hydrogen peroxide, c) extracting hydrogen peroxide withwater to obtain extracted aqueous hydrogen peroxide solution, d)stabilizing the extracted aqueous hydrogen peroxide solution, e)concentrating the aqueous hydrogen peroxide solution to a concentrationof hydrogen peroxide of at least 50% by weight based on the weight ofthe hydrogen peroxide solution to obtain a concentrated aqueous hydrogenperoxide solution comprising: i) less than 50 wppm alkali metals,alkaline earth metals or combinations thereof in total, irrespectivewhether the alkali or alkaline earth metals are present in cationic orcomplex form; ii) less than 50 wppm of amines having a pk_(B) of lessthan 4.5 or the corresponding protonated compounds in total; and iii) atleast 100 wppm anions or compounds that can dissociate to form anions intotal, said wppm being based on the weight of hydrogen peroxide f)drying the working solution after extracting hydrogen peroxide, and g)regenerating and purifying the working solution, whereby during theentire process neither alkali or alkaline earth metals nor amines havinga pk_(B) of less than 4.5 or compounds forming such amines during theprocess are introduced in amounts that result in amounts of i) 50 wppmor more of alkali metals, alkaline earth metals or combinations thereofin total, irrespective whether the alkali or alkaline earth metals arepresent in cationic or complex form; or ii) 50 wppm or more of amineshaving a pk_(B) of less than 4.5 or the corresponding protonatedcompounds in total; in the resulting aqueous hydrogen peroxide solution,said wppm being based on the weight of hydrogen peroxide.
 5. The processof claim 4, wherein the working solution is essentially free of organicnitrogen compounds, drying the working solution in step f) is conductedwithout using alkali or alkaline earth metal compounds, and regenerationof the working solution in step g) is done by treating with activealuminum oxide.
 6. The process of claim 5, wherein drying is conductedby water evaporation in vacuum.
 7. The process of claim 5, wherein nofurther purification of the extracted aqueous hydrogen peroxide solutionis carried out.
 8. The process of claim 6, wherein no furtherpurification of the extracted aqueous hydrogen peroxide solution iscarried out.
 9. The process of claim 4, wherein at least one base havinga pk_(B) of at least 4.5 without containing alkali or alkaline earthmetals is added in an amount resulting in at least 100 wppm of suchbases or corresponding protonated compounds in total based on the weightof hydrogen peroxide in the final aqueous hydrogen peroxide solution.10. The process of claim 9, wherein the base is selected from the groupsconsisting of organic amines and amides having a pk_(B) of at least 4.5,organic hydroxylamines having a pk_(B) of at least 4.5, ammonia andhydroxylamine.
 11. The process of claim 10, wherein the base is ammonia.12. The process of claim 9, wherein the base is added either during thepreparation of the hydrogen peroxide solution or at any stage betweenpreparation and final use of the hydrogen peroxide solution.